The present invention relates to a thermal energy management system, and more particularly, to a thermal energy storage and transfer assembly for gathering radiant thermal energy and kinetic energy of electrons, such as within an electron beam generating device.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. Further, to accelerate the electrons, a typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
As mentioned above, many of the incident electrons are not converted to x-rays, and are deflected away from the target in random directions. For example, up to about 50 percent of the incident primary electrons are back scattered from a tungsten anode target. These back scattered electrons travel on a curvilinear path through the electric field between the cathode and anode until they impact another structure. These electrons interact with the electric field and space charge, causing their initial trajectories to be altered in a complicated, but predictable, manner. The electrons back scatter and bounce off of the internal components of the x-ray tube, transferring kinetic energy, until all of their energy is depleted. In addition to depositing thermal energy into tube components, the impact of back scattered electrons also produces additional off-focal x-rays. This production of off-focal x-ray radiation degrades the image quality if it is allowed to exit the vacuum vessel x-ray transmissive window.
The path of the off-focal radiation and the back scattered electrons may be influenced by the electrical potential configuration of the x-ray tube. In a bi-polar configuration, the cathode is maintained at a negative potential and the anode at a positive potential relative to ground, thereby comprising the total voltage drop across the cathode to anode gap. In this configuration, a large fraction of the initially back scattered electrons from the anode are drawn back to the anode by the electrostatic potential. On the other hand, in a uni-polar design the anode and vacuum vessel are grounded and the cathode is maintained at a high negative potential. In the uni-polar configuration, the back scattered electrons are not drawn back to the anode or attracted to the frame. Therefore, in a uni-polar configuration, a larger fraction of the back scattered electron energy can be beneficially collected and not allowed to return to the anode, thus greatly enhancing the thermal performance of the anode and decreasing the amount of off-focal radiation exiting through the transmissive window.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700.degree. C., while the temperature in the other parts of the anode may range up to about 1800.degree. C. Additionally, the components of the x-ray tube must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450.degree. C. for a relatively long duration.
To cool the x-ray tube, the thermal energy generated during tube operation must be transferred from the anode through the vacuum vessel and be removed by a cooling fluid. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bi-polar configuration. The performance of the cooling fluid may be degraded, however, by excessively high temperatures that cause the fluid to boil at the interface between the fluid and the vacuum vessel and/or the transmissive window. The boiling fluid may produce bubbles within the fluid that may allow high voltage arcing across the fluid, thus degrading the insulating ability of the fluid. Further, the bubbles may lead to image artifacts, resulting in low quality images. Thus, the current method of relying on the cooling fluid to transfer heat out of the x-ray tube may not be sufficient.
Similarly, excessive temperatures can decrease the life of the transmissive window, as well as other x-ray tube components. Due to its close proximity to the focal spot, the x-ray transmissive window is subject to very high heat loads resulting from thermal radiation and back scattered electrons. These high thermal loads on the transmissive window necessitate careful design to insure that the window remains intact over the life of the x-ray tube, especially in regard to vacuum integrity. The transmissive window is an important hermetic seal for the x-ray tube. The high heat loads cause very large and cyclic stresses in the transmissive window and can lead to premature failure of the window and its hermetic seals. Further, as mentioned above, direct contact with the cooling fluid can cause the fluid to boil as it flows over the window. Also, direct contact with a window that is too hot can cause degraded hydrocarbons from the fluid to become deposited on the window surface, thereby reducing image quality. Thus, this solution to cooling the transmissive window may not be satisfactory.
In addition to the thermal effects of back scattered electrons, they can also diminish image quality via the production of non-diagnostic off-focal radiation. Also, x-rays produced by back scattered electrons have a much lower energy spectral content that is not diagnostically beneficial and adds to the patient radiation dose. Thus, it is desirable to prevent the unnecessary x-ray dose of off-focal x-rays from reaching the patient.
The prior art has primarily relied on quickly dissipating thermal energy by using a circulating, coolant fluid within structures contained in the vacuum vessel. The coolant fluid is often a special fluid for use within the vacuum vessel, as opposed to the cooling fluid that circulates about the external surface of the vacuum vessel. Other methods have been proposed to electromagnetically deflect back scattered electrons so that they do not impinge on the x-ray window. These approaches, however, do not provide for significant levels of energy storage and dissipation.
Additionally, these approaches become even more problematic when combined with new techniques in x-ray computed tomography, such as fast helical scanning, that require vastly more x-ray flux than previous techniques. Due to the inherent poor efficiency of x-ray production, the increased x-ray flux is purchased at the expense of greatly increased heat load that must be dissipated. As the power of x-ray tubes continues to increase, the heat transfer rate to the coolant may exceed the heat flux absorbing capabilities of the coolant.
Additionally, these methods do not greatly reduce off-focal radiation or the back scattered electron heat load on the anode. A previous device utilizes an anode hood structure to collimate off-focal radiation. This device has the serious drawback that it relies on radiative cooling and would typically have to operate at very high temperature to transfer the absorbed back scattered electron energy. Other methods employ convection devices which circulate a coolant fluid through a shield within the vacuum vessel. In addition, fluid-cooled shrouds that cover rotating anodes have been used to absorb heat. These approaches rely on thin-walled metal structures to absorb thermal energy and immediately transfer the energy out of the system through a circulating fluid. These methods, however, disadvantageously result in the coolant being subjected to very high heat fluxes and possibly to boiling. Boiling heat transfer is very complicated and can result in high fluid pressure drops. Also, typical prior art devices have high incident heat fluxes, which may result in extreme localized temperatures that may lead to melting of the thin-walled structure and failure of the x-ray tube. Therefore, it is desirable to provide a thermal energy transfer assembly that overcomes the above-stated problems.